For this I intended to use RTRR over FTP. The very first sync worked just fine also if I add new files those are perfectly synced to the Fritzbox. However, renaming files (e.g., folders) on the QNAP seems not to work fine. When I rename a folder, the folder on the Fritzbox is removed, but not added with the new name.
I solved my problem. openvpn file was corrupted by the transfer FTP between my pc and the fritz! box. I try to make a vpn between my fritz! Box and a SIP telephone snom SP 820. This phone is a client vpn openvpn which should allow me to secure and troubleshoot NAT when using a SIP client from the internet. Sylvain